I've heard MacArthur say that no baby (while still a baby) is or was ever condemned to hell, but that all babies that die are saved.
I am not surprised, MacArthur is one of the most sound Teachers in our day, as far as I am concerned.
I think if we consider the teaching of Scripture in it's totality we see a general pattern for God judging men based on the revelation provided to them, their understanding of that revelation, and what they do with it. While it is popular to think that anyone who has never heard the Gospel and accepted it will go to Hell, we look to Paul's teaching concerning the revelation of God in Creation, the hearts of men, and the Word of God. Now Gentiles that did not have the Law can be seen to be said to have fulfilled it, despite not having specific revelation. So too with those that have never heard the Gospel, they will neither be excused nor abandoned because they are not completely without revelation, namely, that of Creation and the internal witness of God given to all men. Those that have received specific revelation will be judged according to their response, whether accepting or rejecting. Those that have rejected the Gospel of Christ as it has been revealed under New Covenant conditions (as opposed to the prophecy of Christ in the Old Testament that, while spoken of, was not revealed in specifics) will be held more accountable than those under the Law, because they have specific revelation concerning Christ, meaning their rejection is not through ignorance or misunderstanding, but because it is the Holy Spirit Who has revealed to them...reject Christ understanding that which they reject.
Going back to babies, it would seem reasonable that God would be just in judgment of those that have not had opportunity to understand to the point where they are held culpable. This would, in my view, extend also to those who may be mentally incapacitated.
I believe firmly that God has made provision for babies just as He has done consistently for all men throughout the Ages, judging justly and according to the response men have given to the revelation of Himself provided to them.
The same is true for us as Christians, not in the eternal sense, but in regards to reward: we are all held to accountability for the revelation of God to ourselves, in our own understanding, and according to our response to that revelation. We have the possibility of obedience, just as we have the possibility of disobedience, but, one thing we can comfort ourselves in, being children of God, is that the One that demands parents to train up their children in the way they should go will not fail, as our Heavenly Father, to that very thing for His children.
God bless.